If your lucky enough to have an all tube stack,then you shouldn't need an overdrive pedal.drive pedals are for us solid state losers.Alot of guitarists are gear crazy(including me) and will buy effects and distortion pedals they dont need or use.
Chuck Shuldiner once said to the effect "If you need lots of effects and pedals to get a good tone,then theres something wrong with your amp".If you NEED a overdrive pedal,then by all means obtain one,but if you get sweet sustain and plenty of roar from the amp then why try to fix something thats not broken.

Later! \m/